The present invention generally concerns treatment of thin films and in particular coloration of anodes for thin film electrochromic devices.
Electrochromic materials have the property of changing its color depending on the charging state. Such materials are today being developed for a number of different applications, ranging from eyewear to displays and smart windows. Nickel oxides and chromium oxides are anodically coloring electrochromic materials that can be used in electrochromic devices together with cathodically coloring WO3, e.g. according to the U.S. Pat. Nos. 5,080,471 and 5,707,556.
An example of such a thin film device is comprised by layered assemblies of glass, WO3, NiOx,Hy, and glass, where the glass is separated from the metal oxides by transparent electron conductors and the metal oxide layers are separated by an ion conductor. The device Us colored and bleached reversibly upon charge transport between the cathode (WO3) and the anode (NiOxHy). Optionally, more layers may be incorporated into the device for different purposes, such as separation of two chemically incompatible layers, optical reflectance optimization etc. The principle of the charge transport in an electrochromic device is the same as in a rechargeable thin film battery.
The electrochromic devices can be assembled in different ways. One way is to build the device by thin film techniques layer by layer. Another possibility is to provide the anode and cathode and subsequently laminate them into a device. In either of these ways, the metal oxide films are provided in either a colored or bleached state before the subsequent lamination or depositing of next layers, see e.g. xe2x80x9cSputter-deposited nickel oxide for electrochromic applicationsxe2x80x9d by A. Azens et. al. in Solid State Ionics vol. 113-115, 1998, page 449-456. The colored state corresponds to pre-charged anode and cathode. If the oxide films are provided in a bleached or transparent state, the final electrochromic device needs a higher operation potential to accomplish the requested charge transfer for the transition into a colored state, which may cause a faster degradation of the device. However, the procedures of laminating or depositing of bleached oxide films are far easier to perform than corresponding procedures for colored oxide films. In practice, lamination in the colored state is today used more often.
One way of pre-charging both the cathode and the anode prior to lamination in a device or depositing the next film layer onto it, is to color the films electrochemically in a liquid electrolyte. However, such methods are provided with inherent disadvantages. The films are spontaneously bleaching upon washing the film after the electrolyte treatment. There are also cumbersome problems to wash away all traces of electrolyte from the surface of the film, which may lead to contact problems in subsequent manufacturing steps, or to an uneven coloration over the surface of the film. The electrochemical treatment also involves a number of process steps, which makes the method time consuming.
Much effort has been focused on dry coloration methods. For WO3, a method of dry lithiation has been developed, consisting of the incorporation of lithium into WO3 upon deposition by e.g. co-evaporation. See e.g. xe2x80x9cHandbook of Inorganic Electrochromic Materialsxe2x80x9d, by C. G. Granqvist, Elsevier, Amsterdam, 1995, pages 79-81. Colored (brown) nickel and chromium oxide films can be made by sputter-deposition in Ar/O2 atmosphere. However, the depth of the coloration obtained in this way for reasonably thin films is not sufficient for subsequent lamination into devices.
One object of the present invention is to provide a method of fabrication of thin film devices, where a coloration/bleaching of metal oxides is achieved in a simple and dry manner. Another object of the present invention is to provide a method of fabrication, which provides thin film devices with an even coloration level. A further object of the present invention is to provide a coloration/bleaching method suitable for both lamination and subsequent film deposition.
The above objects are achieved by the method according to the present invention as defined in the accompanying claims. In general, a color-modifying method according to a first aspect of the invention comprises the provision of a metal oxide thin film based on Ni and/or Cr. The method is characterized in that, when coloring the metal oxide film, exposing the metal oxide thin film to ozone, and when bleaching the metal oxide film, exposing the metal oxide film to UV radiation. A lamination process or a deposition of another thin film may follow the exposure. Preferably, exposing the metal oxide thin film to ultraviolet radiation in an oxygen-containing atmosphere provides the ozone exposure and exposing the metal oxide thin film to ultraviolet radiation in an oxygen-free atmosphere provides the ultraviolet exposure. Suitable metal oxides are selectable among the following oxides: NiCrzOx, NiCrzOxHy, NiVzOx, NiVzOxHy, CrOx, CrOxHy, NiqCrOx, and NiqCrOxHy, where x greater than 0, y greater than 0, 0xe2x89xa6zxe2x89xa61 and q less than 1. A coloring method according to a second aspect of the invention is characterized by exposing the metal oxide thin film to ozone. A bleaching method according to a third aspect of the invention is characterized by exposing the metal oxide thin film to ultraviolet radiation in an oxygen-free atmosphere.
The above methods may be applicable to general thin film devices, but is particularly suitable for electrochemical devices.
The advantages with the present invention are mainly that coloring/bleaching is obtained in a dry state, which is both simpler and cleaner than by electrochemical methods, and that the pre-charging is easily controllable, homogeneous and reproducible. Further advantages with the present invention will be understood by reading the detailed description of embodiments.